Certain optical media, including at least some silica-based optical fibers, can be modified by exposure to electromagnetic radiation in an appropriate spectral range. (Such radiation, typically ultraviolet radiation, is referred to below as "actinic" radiation.) That is, exposure of a photosensitive optical fiber (or other optical medium) to actinic radiation may cause the refractive index to change in the exposed portion of the medium. A periodic pattern can be imposed on the impinging radiation by, e.g., superimposing a pair of beams of substantially monochromatic radiation from, e.g., a laser, to create an interference pattern. When such a patterned radiation field impinges on an optical fiber or other optical waveguide having a core of the appropriate photosensitivity, a corresponding pattern is imposed on the core in the form of periodic (or quasiperiodic) fluctuations in the core refractive index. Such a pattern, which is often referred to as a "Bragg grating" or a "distributed Bragg reflector (DBR)" can behave as a spectrally selective reflector for electromagnetic radiation. Bragg gratings formed in this manner are particularly useful as end-reflectors in optical fiber lasers. These Bragg gratings are useful both because they are spectrally selective, and because they are readily incorporated in the same optical fiber that supports the active laser medium.
A technique for creating these Bragg gratings is described in U.S. Pat. No. 4,725,110, issued to W. H. Glenn, et al. on Feb. 16, 1988, and U.S. Pat. No. 4,807,950, issued to W. H. Glenn, et al. on Feb. 28, 1989. An optical fiber laser having a DBR-terminated cavity is described in G. A. Ball and W. W. Morey, "Continuously tunable single-mode erbium fiber laser", Optics Lea. 17 (1992) 420-422.
Bragg gratings are useful as passive optical components for other applications besides end-reflectors in fiber lasers. For example, Bragg gratings are useful as spectral filters for wavelength-division multiplexing and other optical signal-processing applications. An optical filter which comprises a Bragg grating formed in an optical fiber is described in U.S. Pat. No. 5,007,705, issued to W. W. Morey, et al. on Apr. 16, 1991.
We have observed that when a pair of intersecting laser beams is used to form a Bragg grating in an optical fiber, the resulting grating may exhibit certain optical properties that are generally undesirable. Specifically, the reflectivity spectrum of the grating may exhibit one or two relatively sharp subsidiary peaks, or a regularly spaced sequence of such peaks, to one side of the central peak, generally the short-wavelength side. (These subsidiary peaks are hereafter referred to as "fine structure".) This fine structure is undesirable, for example, in a feedback stabilization system in which the output wavelength of a laser is locked onto the central peak of a Bragg grating. If the grating has subsidiary peaks, it is possible for the tuning of the laser to shift to a subsidiary peak in response to an environmental disturbance. Thus, the presence of fine structure may make a laser system of this kind less robust against environmental disturbances.
By way of illustration, FIG. 1 shows an experimentally measured transmissivity spectrum of a typical Bragg grating formed in an optical fiber. (In the absence of loss, the sum of transmissivity and reflectivity is 100%.) The spectrum includes a broad main peak 10 and a series of subsidiary peaks 15.
We attribute this fine sideband structure to interference effects related to the average axial profile of the refractive index in the grating region. (By the "axial" direction is meant the propagation direction of electromagnetic radiation in the grating.) That is, the refractive index of the fiber (or other waveguiding medium) in the grating region is conveniently described in terms of a perturbation .delta.(z) which represents the difference between this index and the refractive index of the unexposed fiber, and in terms of the variation of the perturbation along the axial direction (i.e., the z-direction). The perturbation varies periodically, in step with the successive light and dark fringes in the interference pattern that created it. However, each of the interfering beams has a spatially varying intensity profile in the plane perpendicular to the propagation direction of the beam. This profile is typically Gaussian in shape. The intensity profiles of the interfering beams define the spatial extent of the Bragg grating, and modulate the amplitude of the periodic refractive index perturbation. As a result, the perturbation .delta.(z) generally takes the form of a periodic series of peaks enclosed by an envelope, typically Gaussian in shape, which is maximal at or near the center of the grating, and falls off to zero at the edges of the grating. If the perturbation is averaged over an axial distance much larger than the grating period, e.g., over ten or more periods, then the resulting average perturbation will of course have the same shape as this envelope.
The existence of such envelopes is well known. In fact, it is well known that an envelope having, e.g., a rectangular shape will give rise to side lobes in the resulting reflectivity spectrum. (See, e.g., H. Kogelnik, "Filter Response of Nonuniform Almost-Periodic Structures", The Bell System Technical Journal 55 (1976) 109-126.) However, the basic reason for this effect is that the grating has a limited spatial extent. By contrast, the fine structure discussed above is a consequence of the spatially averaged perturbation. This average perturbation is effective in certain respects as a "background" perturbation, which has physical significance separate from that of the rapid modulations (i.e., the "lines") of the grating. Until now, a full discussion of the effects of the average perturbation on spectral structure has not appeared in the relevant technical literature. In particular, practitioners in the an have hitherto failed to address possible techniques for mitigating (or for enhancing) the resulting fine structure.
It will be noted that fine structure will ordinarily not be resolved on a conventional optical spectrum analyzer. Such analyzers typically have a resolution, at best, on the order of 1 angstrom. Thus, conventionally measured spectra often fail to illustrate fine structure which is nonetheless present in the waveguide.
In order to accurately resolve fine structure, it is often necessary to employ a high resolution measurement technique such as a tunable external cavity semiconductor laser and power detector which can be stepped in wavelength increments significantly smaller than 1 angstrom, on the order of tenths of angstroms or less. Such a technique can be performed with an HP external cavity tunable laser source model #8168A.